1. Field of the Invention
The present invention relates to on-line color control in printing presses for both process and non-process (PMS or special) colors and, in particular, to the direct utilization of spectral measurements in combination with linear equations to determine the ink flow corrections necessary to maintain accurate color on press.
2. Discussion of the Related Art
Accurate color control of printing presses requires that color deviations between an established color target and a corresponding test area in subsequently printed sheets be kept within established color tolerances. When the color tolerances are exceeded, inking adjustments in the form of solid ink density or ink layer thickness corrections are used in order to reduce the color deviation such that the color-difference is within tolerance.
During printing, it is common practice for a press operator to visually monitor the printed images and adjust the flow of ink into the press until a visual match is achieved. A pre-press proof or previously printed xe2x80x9cColor OKxe2x80x9d sheet is typically used as the aim or target condition. Due to the inherent variation in observers color vision, both within individuals over time and between different individuals, this procedure is subject to large variability and is also time consuming. Instrumental color control offers an alternative for process color control that is more repeatable, accurate and efficient.
Within the graphic arts industry, densitometry has been the main measurement method for measuring and controlling the primary inks and related attributes in process color printing. Although the densitometer is suited for measurements pertaining to the relative strength of a process color solid ink film, the densitometer is not capable of describing the color of an object in a manner that relates to the visual sensation of color. Color control applied to the control of a printing press has as one of it""s main purposes the task of maintaining a visual match between an established color target location and that same location in subsequently printed sheets such that no visually disturbing color differences appear throughout the duration of the press run. Therefore, a color measurement instrument that is capable of describing the color of objects in approximate visual terms is required. Instruments that are capable of this requirement include calorimeters and spectrophotometers. Spectrophotometers, furthermore, have the advantage that they can report both densitometric and colorimetric data calculated according to standard procedures.
Measurements for color control are most commonly made on color control bars that contain a variety of test elements, which provide information on print quality attributes. Although color control based on color bar measurements provides a high level of print quality, it is alternatively possible to attain a high level of print quality by measuring within the image. In such cases, color control based on intra-image measurements, either in combination with a color bar or without, can provide the necessary level of print quality.
Control of any system requires knowledge of the relationship between the input variable(s) and the output variable(s). In printing, although there are many options for input variables, the main press control or output variable influencing the visual impression of the printed image is the inking system, which modulates the flow of ink into the press. By varying the volume of ink flowing into the press, the thickness of the ink layer deposited onto the paper will vary, thereby influencing the color of the print.
Although, multi-color halftone image reproduction is in general a non-linear process, under certain conditions it is possible to use linear equations to model the process by restricting the range of the transformation to a sub-region of the color gamut. Within each sub-region, which has the target color as its origin, a set of xe2x80x9clocalizedxe2x80x9d equations can be used. The region over which the localized transformations will be linear is dependent on the target color location and the input and output variables used to represent the differences between the test and target areas in the transformation.
Methods for performing color control on printing presses using a spectrophotometer are described in U.S. Pat. Nos. 4,975,862, 5,182,721 and 6,041,708. These patents however, describe methods for controlling the printing press with colorimetric coordinates, which are obtained from spectral reflectance data, rather than using the spectral reflectance data directly.
Several aspects of the aforementioned U.S. patents can be improved upon for use in a color control system. Colorimetric models provide less accurate control as compared to spectral models primarily in situations where the spectral reflectance difference between two ink settings cannot be described by a single constant or multiplication factor. Additionally, off-line methods of calculating the parameters of the matrix relating solid ink density or ink layer thickness differences to spectral reflectance differences are not accurate enough for use in a commercial color control system. Such methods only represent the state of the system at one point in time. Dynamic methods of calculating the matrix on-line in real-time during the press run would greatly improve the effectiveness and accuracy of the control method.
The present invention provides a method of color control for printing presses through direct use of spectral reflectance data. Spectral reflectance differences between a target and test area are determined and used to calculate solid ink density or ink layer thickness corrections for use in controlling the printing press. The method converts a spectral reflectance difference directly into either solid ink density or ink layer thickness corrections through the use of a linear equation employing an empirically derived transformation matrix, which is calculated on-line. The method is applicable to the control of both process and non-process (PMS or special) colors.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized.